The isoforms of vertebrate sarcomeric myosin heavy chains (MHC) are encoded by the members of a multigene family. Adult skeletal muscles contain a heterogeneous population of fibers which can be characterized by these MHC isoforms. These fibers can switch from the synthesis of one MHC isoform to the synthesis of another, a transformation which involves activation of one MHC gene and the repression of another and which is controlled both by the pattern of neural stimulation and the levels of several hormones. During development, there is a sequential synthesis of several stage-specific isoforms prior to the synthesis of the cell's adult MHC isoform. This family of proteins,however,is more than a marker for change. The proteins themselves, because they define the speed of contraction of the muscle fiber and because they are coordinately controlled with the ATP generating system might themselves be implicated in the genesis or symptomotology of muscle diseases such as floppy infant syndrome, respiratory muscle fatigue, or hypo- and hyperthyroidism. Our long term goal is to define the factors controlling expression of MHC isoforms and to investigate hoe the regulation might go awry in human neuromuscular diseases. For the next few years,we plan to define the organization of human MHC genes on chromosome 17, to determine their expression during human development, and to relate regulatory sequences around each MHC gene with that gene's tissue-specific and stage-specific control. To accomplish these goals,we have proposed the following specific aims: 1) to isolate and sequence cDNAs representing the coding region of each human MHC gene; (@) to prepare isoform-specific monoclonal antibodies; (3) to use the isoform-specific monoclonal antibodies and cDNAs to analyze the developmental expression of each human MHC gene; (4) to isolate genomic DNA spanning and flanking the putative MHC gene cluster on chromosome 17; (5) to determine the organization of MHC genes in the MHC gene cluster; (^) to use 5' non coding regions from the MHC genes in transfection experiments to identify regions which can be implicated in tissue-specific of stage-specific regulation. Accomplishment of these goals will be important to the future understanding of the control of contractile protein genes in human neuromuscular diseases.